Arare smidgen of ancient DNA has sharpened the picture of one of humanity’s greatestmigrations. Some 15,000 to 25,000 years ago, people wandered from Asia to NorthAmerica across a now-submerged land called Beringia, which once connectedSiberia and Alaska. But exactly when these ancient settlers crossed and howmany migrations occurred are hotly debated. Now, the oldest full genome to besequenced from the Americas suggests that some settlers stayed in Beringiawhile another group headed south and formed the population from which allliving Native Americans descend.

“Thisis an important study that significantly narrows the subset of possibilities[for how the Americas were peopled],” says David Reich, a geneticist atHarvard Medical School in Boston. “It’s very exciting.”

Thegenome comes from an 11,500-year-old infant found in 2013 at the site of UpwardSun River in central Alaska’s Tanana River Basin, a part of Beringia that’sstill above sea level. The infant, one of two from the site, belonged to apopulation that likely numbered in the low thousands, who hunted Beringia’s abundantherds and gathered plants.